1. Field of the Invention
The present invention relates to a method of holding a mask used in an exposure apparatus.
2. Description of the Related Art
The integration density of semiconductor integrated circuits has recently increased, and, in a semiconductor producing apparatus for producing such highly integrated circuits, printed wires have become finer with the increase in integration density, and therefore, higher exposure precision is required. In order to make the printed wires finer, it is effective to shorten the wavelength of a light source used for exposure. An X-ray exposure apparatus using X-rays having a shorter wavelength than that of ultraviolet rays, which are generally used, has been developed.
FIGS. 9(a) and 9(b) are schematic views showing an X-ray mask and a mask chuck mounted in a conventional X-ray exposure apparatus. FIG. 9(a) illustrates an X-ray mask which comprises a reinforcing mask frame 100, a mask substrate 101 made of silicon, an inorganic film (mask membrane) 102 formed by removing a portion of the mask substrate 101 by back etching, and a transfer pattern 103 of a semiconductor circuit and the like drawn on the mask membrane 102 by an electron beam (EB) drawing machine or the like. A magnetic ring 105 made of a magnetic material is embedded in the mask frame 100.
FIG. 9(b) illustrates a magnetically attractive mask chuck. A hole 111, through which exposure X-rays pass, is formed inside a ring-shaped chuck base 110. A magnetic unit 112 is circumferentially disposed corresponding to the magnetic ring 105, and generates a sufficient magnetic force to attract and hold the X-ray mask. These structures of the X-ray mask and the mask chuck allow the mask frame 100 of the X-ray mask to be magnetically attracted to and held in planar contact with a holding plane of the chuck base 110. It is noted that vacuum attraction may be used instead of the above magnetic attraction. When using vacuum attraction, the magnetic unit 112 is replaced with a vacuum port, and the mask frame 100 and the chuck base 110 are brought into planar contact with each other, and attracted by vacuum force.
However, since the mask frame 100 and the chuck base 110 are in planar contact with each other in these attractive holding methods, contact planes thereof are required to have a highly flat finish. If an attracted plane of the mask frame 100 has even a small amount of distortion, such as a warp, at the time when the transfer pattern is formed by the EB drawing machine during production of the mask, when the mask frame 100 is held by the chuck base 110 of the X-ray exposure apparatus, it is deformed by correcting its warp. There is a possibility that the stress resulting from the deformation will be transmitted to the transfer pattern 103 through the mask substrate 101 and the mask membrane 102, and the transfer pattern 103 will become more distorted than it is when being drawn. The thickness of the mask membrane 102 on which the transfer pattern 103 is formed is approximately 2 .mu.m, and its rigidity is much smaller than those of the mask substrate 101 and the mask frame 100, each being a few millimeters in thickness. Therefore, distortion of the mask substrate 101 and the mask frame 100 has a great influence on the mask membrane 102, and also distorts the transfer pattern 103 badly. This problem is peculiar to a mask in which a transfer pattern is formed on an extra-thin mask membrane.
In order to solve the above problem, a method for preventing an X-ray mask from being deformed by a holding force when being held on a mask chuck, in other words, a method for holding the X-ray mask while keeping distortion of a mask frame caused in forming a transfer pattern (referred to as the "kinematic mount" hereinafter) has been suggested.
FIGS. 10(a) and 10(b) are views showing an example of a kinematic mount. FIG. 10(a) illustrates an X-ray mask for a kinematic mount. A conical (funnel-shaped) hole portion 117, a planar portion 118 and a V-groove portion 119 in which a cutting groove is linearly formed in the X direction in FIG. 10(a) are formed on a holding plane of a mask frame 100. FIG. 10(b) illustrates a mask chuck for a kinematic mount. Spherical projections 120 are disposed at three positions on a holding plane of a chuck base 110 to respectively engage with the conical hole portion 117, the planar portion 118 and the V-groove portion 119 of the mask. Furthermore, clamp mechanisms 115 for mechanically pressing and holding the mask are mounted at the three positions.
This structure brings the following holding state in which the mask equipped with six degrees of freedom is positioned without being overconstrained.
______________________________________ Restrained Free ______________________________________ conical hole portion 117 X, Y, Z -- planar portion 118 Z X, Y V-groove portion 119 Y, Z X ______________________________________
According to the kinematic mount, since little external force for deforming the mask frame 100 acts on the mask frame 100 while the mask is held during exposure and the mask can be held (without distortion) in the same state as when the mask pattern is formed by the EB drawing machine, the mask pattern can be prevented from being distorted by deformation of the mask frame 100.
However, in the above kinematic mount, if the mask expands or shrinks with heat while being held by the mask chuck, since only the position of the conical hole portion 117 does not change and the planar portion 118 and the V-groove portion 119 are displaced relative to the conical hole portion 117 on an X-Y plane, positional changes owing to expansion and shrinkage cannot be relieved evenly. Therefore, the expansion and shrinkage of the mask frame have a great influence on pattern transfer accuracy.
Furthermore, since the three portions for supporting the mask are different in shape, it takes a significant amount of labor in manufacture and dimension control. In particular, the conical hole portion 117 requires a high precision in finishing and dimension control, and therefore, there is an overall problem in increased costs of mass-production.